5.4Heat Stress and Root---Soil Inte

5.4Heat Stress and Root---Soil InteractionsIn the previously described study with big bluestem (A. gerardii) [80], we observedan increase in the rhizosphere concentration of recently fixed plant C duringheating and hypothesize that this was likely due to increased exudation or fine-rootturnover (the latter during severe rather than moderate heat stress), rather thandecreased microbial uptake, as heat: (i) increased the concentration of translocatedshoot 13C in both rhizosphere (but not bulk) soil and roots, (ii) did not increasemicrobial respiration, but increased microbial biomass, and (iii) did not change soiltotal C or N concentration. As root exudation of soluble sugars and proteindecreased with heating in the lab experiment, then fine-root loss is the more likelyexplanation for the increase in plant C loss to soil with heating. Also, a largeincrease in soil 13C in the t10 versus t5
C heat treatments, with little differencein microbial respiration or biomass between these two treatments, indicates that Closs from roots to soil increases with temperature. Since neither shoot mass norphotosynthesis declined significantly with heating, an increase in fine-rootturnover was not likely due to declines in whole-plant net photosynthesis (thoughtotal shoot C flux was not measured). Rather, we hypothesize that fine-root turnoverincreased either because of the increase in root growth during heat stress orbecause increased transpiration during heat induced localized water stress in fineroots (e.g., in the upper 10-cm portion of the soil). If this hypothesis is correct, thenwe predict that there will be differences in heat effects on root C loss between heat?tolerant species and -sensitive species, since heat-tolerant species can bettermaintain root growth and transpiration during heat. The above results also suggest that the increase in microbial biomass is “fueled” by the release of root C to the soil,but this hypothesis will need to be tested by determining if plant C is being takenup by soil microbes. Finally, the results suggest that the decrease in root functionduring heat is largely, but not entirely, due to indirect effects (since root/soilrespiration decreased more in the lab, where soil temperature increased, versus thefield, where soil temperature increased little at 10-cm depth) and the decrease inroot function might be due to decreases in labile C resulting from increased rootgrowth (i.e., does the increase in root mass, to increase water uptake, duringheating come at the expense of non-structural C, which decreases root function pergram).